Actuator and brake device

ABSTRACT

An actuator has an electric motor, a screw shaft connected to a rotational shaft of the electric motor in such a manner as to be capable of transmitting power, a nut member disposed on peripherally of the screw shaft and connected to the rotational shaft of the electric motor in such a manner as to be capable of transmitting the power and a ball rolling within a spiral groove formed between the screw shaft and the nut member, characterized in that the rotational speed of the screw shaft and the rotational speed of the nut member are different.

TECHNICAL FIELD

The present invention relates to an actuator and more particularly to anactuator preferable for use in, for example, an electric brake system ofa vehicle and a brake system that uses the actuator.

BACKGROUND ART

Currently, disc brake systems used in many passenger cars and the likeare such that a pad is pressed against a disc rotor by making use offluid pressure that is generated by a master cylinder or the like whichis made to interlock with a brake pedal so as to apply the brake. Inparticular, in recent years, boosters have been put into practical usein which the braking force is increased by making use of negativepressure, compressed air, fluid pressure and the like in addition tohuman power.

In the disc brake systems which make use of fluid pressure and the like,however, piping is necessary for fluid, which needs, in turn, designsfor avoiding the interference with other components and calls for a costincrease. Furthermore, in recent years, in association with the spreadof ABS (antilock brake system) and TRC (traction control system), brakesystems have been in demand which are quicker to respond and which canbe controlled more accurately. This background has developed electricbrake systems in which the driving force of an electric motor is used tothrust a piston to press a pad against a disc rotor (refer to JapanesePatent Unexamined Publication JP-A-3-41233).

Incidentally, in the electric brake systems, there may be a case where aball screw mechanism or the like is adopted to convert the rotationalforce of the electric motor into a thrust by a piston (refer to Gazetteof International Unexamined Patent Publication No. 99/60285,JP-A-10-257716).

Here, for example, in the electric brake system, in order to secure asufficient braking force, an extremely large force needs to be securedfor pressing the pad against the disc rotor. In contrast to this, thetorque of an electric motor that can be installed on a vehicle isgenerally small. On the other hand, in the ball screw mechanism, whenthe lead angle of the screw is changed, the reduction ratio can beincreased to some extent, but this generates problems. Then, in a systemdisclosed in the pamphlet of International Unexamined Patent PublicationNo. 99/60285, a large reduction ratio is given to the output of theelectric motor using a bevel gear and a planetary gear mechanism, sothat a large thrust is transmitted to the pad. When the transmissionsystem that uses the bevel gear and planetary gear mechanism, there iscaused a problem that the system becomes bulky as a whole and hence, acompact construction cannot be provided. In addition, when attempting touse a smaller electric motor, the reduction ratio needs to be increasedfurther, this causing a problem that the transmission system becomeslarger in size. In addition, in the system disclosed in JP-A-10-257716,a construction is described in which a single motor is used to rotate ascrew shaft and a nut, and a spline groove is cut in the screw shaft,whereby the screw shaft is rotated by rotating a nut on the spline. Theinvention is such that the ball screw nut and the spline nut are drivenby the single motor, while normally, two motors are used to rotateseparately the ball screw nut and the spline nut in order to obtain twodegrees of freedom in rotation and axial movement (for example, JapanesePatent Examined Publication 2722345). To make this happen, two sets ofbrakes are incorporated in a speed reducer so as to create a mode forrotating only the nut of the ball screw and a mode for rotating both thenuts. This requires a mechanism which uses a number of components andwhich is expensive and a sophisticated control. Since an actuator of thebrake system does not need to rotate, an inexpensive and compactconstruction is desired which lacks a control for rotational directions.

The present invention was made in view of the problems inherent in theconventional techniques, and an object thereof is to provide an actuatorwhich can exhibit a strong driving force while being kept compact insize and a brake system using the same.

SUMMARY OF THE INVENTION

An actuator of the present invention having an electric motor, a screwshaft connected to a rotational shaft of the electric motor in such amanner as to be capable of transmitting power, a nut member disposed onperiphery of the screw shaft and connected to the rotational shaft ofthe electric motor in such a manner as to capable of transmitting thepower and a ball rolling within a spiral, groove formed between thescrew shaft and the nut member, characterized in that the rotationalspeed of the screw shaft and the rotational speed of the nut member aredifferent.

Since the actuator of the invention has the electric motor, the screwshaft connected to the rotational shaft of the electric motor in such amanner as to be capable of transmitting power, the nut member disposedon the circumference of the screw shaft and connected to the rotationalshaft of the electric motor in such a manner as to be capable oftransmitting power and the balls which roll within the spiral grooveformed between the screw shaft and the nut member and the rotationalspeed of the screw shaft and the rotational speed of the nut member aredifferent. in the event that the nut member is rotated relative to thescrew shaft at a low rotational speed, the nut member moves in an axialdirection according to the relative rotation. Therefore, in the eventthat the amount of the relative rotation is made small, a largereduction ratio can be provided in the axial movement of the nut memberrelative to the rotation of the screw shaft without changing the leadangle of the screw shaft or depending upon other transmission systems.

While a large reduction ratio can be provided when the screw shaft andthe nut member rotate in the same direction, a small reduction ratio(namely, a quick motion of a member to be driven) when they rotate indifferent directions.

A first gear and a second gear, which have different numbers of teeth,are integrally formed on the rotational shaft of the electric motor, anda third gear and a fourth gear are integrally formed, respectively, onthe screw shaft and the nut member, whereby the first and second gearspreferably mesh with the third and fourth gears, respectively.

The facewidth of the first gear and the facewidth of the second gear arepreferably different.

One of the screw shaft and the nut member is made stationary withrespect to the axial direction, whereas the other is allowed to move inthe axial direction, and of the first gear and the second gear, thefacewidth of the gear which meshes with the gear formed integrally onthe member which is allowed to move in the axial direction is preferablylonger than the facewidth of the other gear.

At least either the gear integrally formed on the member which isallowed to move in the axial direction or the gear on the rotationalshaft which meshes with the gear is preferably made from a resin. Wearand noise can be reduced by the gear which is made from a resin.

A member to be driven is preferably mounted on the member which isallowed to move in the axial direction via a thrust bearing. Thefriction loss can be reduced by the thrust bearing.

The member to be driven is a brake pad, and an electric brake system ispreferably such that a caliper is made up of the actuator. Since thegear sets can be accommodated within the caliper, the brake actuator canbe provided which is compact in size and provides a large output(thrust).

It is preferable that one of the screw shaft and the nut member is madestationary with respect to the axial direction, whereas the other isallowed to move in the axial direction. Note that when power istransmitted from the rotational shaft to the screw shaft and the nutmember using the pairs of gears, in the event that the gear on themember which moves axially is made from a resin, high sliding propertiesare preferably provided.

It is preferable to use the actuator described heretofore in a brakesystem.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of an electric brake system containing anactuator according to an embodiment of the invention.

Note that in the drawing, reference numerals denote as follows: 10 ahousing; 11 an electric motor; 13 a gear shaft; 16 a gear; 17 acylindrical member; 18 a screw shaft; 23 a ball; 23A, 23B pads; and 24 adisc rotor.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, an embodiment of the present invention will be described byreference to the drawing. FIG. 1 is a sectional view of a brake systemin which an actuator of a first embodiment is installed.

In FIG. 1, a rotational shaft 11 a of an electric motor 11 mounted on ahousing (making up a caliper) 10 having a U-shaped section which isfixed to a vehicle body, not shown, is connected to an end portion of agear shaft (a rotational shaft) 13 which is supported by bearings 12, 14rotatably relative to the housing 10 at cylindrical shaft portions 13 a,13 b situated at ends thereof so as to rotate together with the gearshaft 13. Note that an opening in the housing 10, in which the gearshaft 13 is provided, is closed with a lid 15. Note that a preload isgiven to the bearings 12, 14 so as to eliminate axial loosening.

A gear portion 13 c (a first or second gear) having a number of teethZ_(A) and a short axial width and a gear portion 13 d (the first orsecond gear) having a number of teeth Z_(C) and a long axial width areformed on a circumferential surface of the gear shaft 13 excluding thecylindrical portions 13 a, 13 b. The gear portion 13 c meshes with agear 16 (a third gear) having a central opening and a number of teethZ_(B), and the gear portion 13 d meshes with a flange-shaped gearportion 17 a (a fourth gear) which is formed on a hollow cylindricalmember 17 and which has a number of teeth Z_(O). The gear 16 is mountedon a larger cylindrical portion 18 a of a screw shaft 18 which is madeto pass through the inside of the cylindrical member 17 using a key 19in such a manner as to rotate together with the cylindrical member 17.

A smaller cylindrical portion 18 b of the screw shaft 18, which isformed on a right side thereof as viewed in the drawing, is supported bya needle bearing 20 rotatably relative to the housing 10. A right sidesurface, as viewed in the drawing, of the gear 16 fixed to the screwshaft 18 is supported by a thrust bearing 21 rotatably relative to thehousing 10. The cylindrical member 17 is supported by a bush 22rotatably and axially movable relative to the housing.

A thread groove 18 c is formed in an outer circumferential surface ofthe screw shaft 18 excluding the cylindrical portions 18 a, 18 b,whereas a thread groove 17 b is formed in an inner circumferentialsurface of the cylindrical member (a nut member) 17 disposed on theouter circumferential surface of the screw shaft 18 in such a manner asto correspond to the thread groove 18 c, and a number of balls 23 aredisposed in a spiral space formed by both the thread grooves 18 c, 17 cin such a manner as to roll therein. Furthermore, although not shown, atube is provided on the outer circumference of the cylindrical member 17for returning the balls 23 from one end to the other end of a transferpath. The cylindrical member 17, the screw shaft 18 and the balls 23make up a ball screw mechanism.

A disc rotor 24, which rotates together with a wheel (not shown), isdisposed in such a manner as to be held by a pad 23A fixed to thehousing 10 and a movable pad 23B, and a pressing member 25 for pressingthe pad 23B against the disc rotor 24 is mounted on a left end of thecylindrical member 17 as viewed in the drawing. A thrust bearing 26 isdisposed between the pressing member 25 and the pad 23B so as tofunction to prevent the transmission of a rotational force of thepressing member 25 which rotates together with the cylindrical member17. Note that the thrust bearing 26 may be disposed between thecylindrical member 17 and the pressing member 25.

The operation of the embodiment will be described. In FIG. 1, whenelectric power is supplied from a power supply, not shown, so that therotational shaft 11 a of the electric motor 11 and the gear shaft 13rotate clockwise, the gear 16, which meshes with the gear portion 13 c,rotates together with the gear shaft 13 counterclockwise, and thecylindrical member 17 having a gear portion 17 a which is in meshengagement with the gear 13 d rotates together with the gear shaft 13counterclockwise. Here, if a (the number of teeth Z_(A)/the number ofteeth Z_(B)) and a (the number of teeth Z_(C)/the number of teeth Z_(D))are different, and the (the number of teeth Z_(A)/the number of teethZ_(B))>the (the number of teeth Z_(C)/the number of teeth Z_(D)), sincethe rotational speed (the number of rotations) of the gear 16 or thescrew shaft 18 is higher than the rotational speed (the number ofrotations) of the cylindrical member 17, there is caused a difference inrotational speed. In case the ball screw mechanism is a right-handthread, the cylindrical member 17 moves leftwards in the axial directionas viewed in the drawing. When the cylindrical member 17 moves leftwardsin the axial direction, the pressing member 25 presses the pad 23Bagainst the disc rotor 24 via the thrust bearing 26, whereby a brakingforce is exhibited. As this occurs, a reaction force of the pad 23B isborne by the thrust bearing 21 via the screw shaft 18 and the gear 16.Note that when electric power having an opposite characteristic issupplied from the power supply, not shown, so that the rotational shaft11 a of the electric motor 11 rotates in the other direction, the pad23B is separated apart from the disc rotor 24 as opposed to theaforesaid movement.

Here, a reduction ratio i of this embodiment is expressed by thefollowing equation:i=1/{(Z _(A) /Z _(B))−(Z _(C) /Z _(D))}  (1)Namely, the nearer the (the number of teeth Z_(A)/the number of teethZ_(B)) and the (the number of teeth Z_(C)/the number of teeth Z_(D)) areto each other, the larger the reduction ratio i becomes, and even whenthe torque of the electric motor 11 is small, a large pressing force bythe pad 23B can be secured. To be specific, when Z_(A)=16, Z_(B)=61,Z_(C)=15 and Z_(D)=62, i=49. Consequently, according to the embodiment,since such a large reduction gear ratio can be obtained, even when asmall torque, high rotational speed motor is used, a large pad pressingforce can be secured without using other transmission devices such as abevel gear and a planetary gear mechanism, whereby a compactconstruction can be provided. However, in case, in addition to theembodiment, a bevel gear and a planetary gear mechanism are provided, alarger pad pressing force can be secured.

Note that the gear 16 and, in particular, the gear portion 17 a of thecylindrical member 17 which moves axially or the gear portion 13 d ismade from a resin, the sliding properties are increased, whereby thewear of the surfaces of the teeth is reduced, and a noise suppressingeffect can be expected. As this occurs, the cylindrical member 17 ispreferably of a hybrid construction of the resin material (the gearportion) and a steel material (the nut portion).

Thus, while the invention has been described heretofore by reference tothe embodiment thereof, the invention is not construed as being limitedthereto, but may, or course, be modified and improved appropriately.

While the invention has been described in detail and by reference to thespecific embodiment heretofore, it is clear to those skilled in the artthat the invention can be changed and modified variously withoutdeparting from the spirit and scope of the invention.

The subject patent application is based on the Japanese PatentApplication (No. 2003-012165) filed on Jan. 21, 2003 and the contentsthereof is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

Since the actuator of the invention has the electric motor, the screwshaft connected to the rotational shaft of the electric motor in such amanner as to be capable of transmitting power, the nut member disposedon the circumference of the screw shaft and connected to the rotationalshaft of the electric motor in such a manner as to be capable oftransmitting power and the balls which roll within the spiral grooveformed between the screw shaft and the nut member and the rotationalspeed of the screw shaft and the rotational speed of the nut member aredifferent, in the event that the nut member is rotated relative to thescrew shaft at a low rotational speed, the nut member moves in an axialdirection according to the relative rotation, and therefore, in theevent that the amount of the relative rotation is made small, a largereduction ratio can be provided in the axial movement of the nut memberrelative to the rotation of the screw shaft without changing the leadangle of the screw shaft or depending upon other transmission systems.

1. An actuator comprising: an electric motor; a screw shaft connected toa rotational shaft of the electric motor in such a manner as to becapable of transmitting power; a first gear portion formed on an entireouter circumferential surface, except for shaft support portions, ofsaid rotational shaft; a nut member disposed on a periphery of the screwshaft and connected to the rotational shaft of the electric motor insuch a manner as to be capable of transmitting the power; a second gearportion directly formed on a flange portion of said nut member; and aball rolling within a spiral groove formed between the screw shaft andthe nut member, characterized in that the rotational speed of the screwshaft and the rotational speed of the nut member are different.
 2. Theactuator as set forth in claim 1, characterized in that the screw shaftand the nut member rotate in the same direction.
 3. The actuator as setforth in claim 1, characterized in that one of the screw shaft and thenut member is made stationary with respect to an axial direction, andthe other is allowed to move in the axial direction.
 4. An actuator asset forth in claim 1, characterized in that a first gear and a secondgear, which have different numbers of teeth from each other, areintegrally formed on the first gear portion rotational shaft of theelectric motor, and a third gear and a fourth gear are integrallyformed, respectively, on the screw shaft and the second gear portion ofthe nut member, whereby the first and second gears mesh with the thirdand fourth gears, respectively.
 5. An actuator as set forth in claim 4,characterized in that a facewidth of the first gear and a facewidth ofthe second gear are different from each other.
 6. An actuator as setforth in claim 5, characterized in that one of the screw shaft and thenut member is made stationary with respect to the axial direction,whereas the other is allowed to move in the axial direction, and in thatof the first gear and the second gear, the facewidth of the gear whichmeshes with the gear formed integrally on the member which is allowed tomove in the axial direction is longer than the facewidth of the othergear.
 7. An actuator as set forth in claim 4, characterized in that atleast either the gear integrally formed on the member which is allowedto move in the axial direction, or the gear on the rotational shaftwhich meshes with the gear formed on the member which is allowed to movein the axial direction, is made from a resin.
 8. An actuator as setforth in claim 3, characterized in that a member to be driven is mountedon the member which is allowed to move in the axial direction via athrust bearing.
 9. An actuator as set forth in claim 8, characterized inthat the member to be driven is a brake pad.
 10. An electric brakesystem characterized in that a caliper is made up of the actuator setforth in claim
 9. 11. A brake system characterized by usage of theactuator set forth in claim
 1. 12. An actuator as set forth in claim 1,wherein said second gear portion is directly formed on an outerperipheral surface of said flange portion of said nut member.
 13. Anactuator as set forth in claim 1, wherein a diameter of said second gearportion is the same as a diameter of said flange portion.
 14. Anactuator as set forth in claim 1, wherein said flange portion isdisposed on said nut member so that there is no relative axial movementbetween said second gear portion and the spiral groove of said nutmember as said nut member and said screw shaft undergo relativerotation.